DE68918989T2 - Corrosion protection. - Google Patents

Abstract

A method of removing iron contaminants in an aqueous system by introducing and maintaining within the system an ortho dihydroxylaromatic compound having at least one electron withdrawing group pendant from the aromatic ring and, further, a method of inhibiting calcium scale formation in the presence of iron contaminants by utilizing a dihydroxylaromatic compound in combination with a calcium scale inhibitor.

Description

The present invention relates to a new process for removing iron contaminants commonly found in aqueous systems. The present process provides a means for expelling and removing ferric scales formed in systems containing iron and/or iron-based alloy components in contact with the aqueous system.

The present invention relates to the use of a substantially non-toxic process for removing corrosion products from ferrous metals in contact with aqueous systems, in which these products are effectively expelled and removed from the system. The process describes and claims the use of at least one adjacent-paired or ortho-dihydroxyaromatic compound which further contains at least one electron-withdrawing group bonded to the aromatic ring.

The present invention is described in terms of its use in connection with cooling water systems. However, the invention is not so limited and can be effectively used in connection with other aqueous systems in contact with iron-based alloy material and particularly in difficult to control systems that present conditions such as elevated pH, high temperatures and/or high hardness levels such as boilers, heat exchangers and the like.

The expulsion and/or removal of iron corrosion formations is essential in cooling water systems to keep the system free from interference/clogging and to create an efficient system by maximizing flow rate and heat exchange. Iron(III) oxide scale is known to build up in these systems and cause a significant decrease in their efficiency. Iron oxide scale is particularly troublesome because it is extremely insoluble in aqueous media. and therefore deposits and builds up in the system, reducing the flow rate and impairing heat exchange. There is therefore a need for additives that are capable of removing the iron scale and keeping the system free from scale buildup. To do this effectively, the additive must be able to dislodge and dissolve the iron oxide solids present in the system.

There are many agents that have been proposed for the removal of iron scale. To be useful under the conditions normally encountered in refrigeration systems and the like, a product must be able to meet the following combination of rigorous criteria:

1) Dissolution of the old rust crust,

2) Dissolution of solids of iron(III) oxides and iron(III) hydroxides, which are materials that are normally insoluble in high pH cooling water conditions and are normally very hard platelets with high calcium and magnesium content,

3) the ability to control the formation of new rust crusts such as iron(III) over an extended period of time,

4) the ability to remove the iron crust from the system without merely expelling the solids from their place of formation since such solids tend to be trapped in other places of smaller dimensions, causing clogging of the system,

5) Use of materials or compositions that are stable under the adverse conditions present such as high pH (6.5 to 9.5), high temperature (e.g. 38 to 79º C (100 to 175º F)) and/or hardness associated with the presence of excess calcium, magnesium and carbonate ions,

6) Use of material capable of remaining soluble under the unfavourable existing conditions,

7) not to be a source of corrosion or acceleration of corrosion of the system,

8) the ability to complex iron ions in low amounts,

9) the ability to increase the performance of the threshold crust inhibitors in the presence of iron, and

10) the ability to promote the formation of protective oxide layers that passivate the metal surface against further corrosion.

It is readily apparent that means to achieve this combination of desired properties would find a high degree of acceptance in the control of iron scale in cooling water systems.

Scale prevention is the primary goal for maintaining a clean system. However, inadvertent increases in pH, temperature, concentration cycles, flow rate, etc. in the system will result in the formation of some iron scale that is not prevented by the normal maintenance doses of conventional additives. There is therefore a need for an additive that will remove iron oxide scale that has formed due to such disturbances.

Classical methods for removing iron scale involve acid or mechanical cleaning. These methods are undesirable because they require expensive shutdown of the system being treated and result in equipment destruction via corrosion and/or mechanical abrasion. A means of removing iron scale during normal operation of the cooling water system would improve economics, simplify operations and minimize equipment destruction.

In order to control the iron crust in cooling water, polymer additives were used in an experiment. For example, In US-A-3 898 037 the dispersion of insoluble iron compounds with polymers of 2-acrylamido-2-methylpropanesulfonic acid has been described. Sulfonated polymers have also been used for this purpose (Proc. Int. Water Conf. Eng. Soc. West. Pa. 1978, 39, 299). These dispersants are able to prevent iron oxide solids from depositing. However, they do not dissolve iron(III) oxides and do not remove hardened or crystalline deposits which are often present in cooling water systems.

Chelating agents have been used to prevent and remove iron oxide scale by sequestration. Ethylenediaminetetraacetic acid has been used to remove iron oxide scale (US-A-2,396,938), but is only effective in the absence of excess calcium, a common condition found in cooling water systems and the like. Its use can also lead to increased corrosion. Organophosphates such as aminotrimethylenephosphonic acid (ATMP) have been shown to sequester iron (III), keeping it in solution up to pH 10 (Dequest, Technical Bulletin 1-247, Monsanto, 1972). However, it is known that these organophosphonates generally do not dissolve iron oxide or iron hydroxide solids in water containing high concentrations of calcium ions at a pH of 8 or higher. These water conditions are typical of normal operation of a cooling water system and the like.

In addition, these organophosphonates have been shown to precipitate at high levels of calcium, which precludes their use for iron scale control in hard water.

JP-A-58133382 describes compounds of the general formula

in which X is hydrogen, alkyl of 1 to 4 carbon atoms, hydroxy, alkoxy of 1 to 4 carbon atoms, or carboxy and Y is hydrogen or hydroxy (provided that when Y is hydroxy, X is something other than hydroxy or alkoxy), as corrosion inhibitors for calcium chloride salt solutions: such salt solution generally contains 15 to 35% by weight calcium chloride. The compounds have been described as suppressing the corrosion of ferrous metals in contact with such salt solution: there is no disclosure or suggestion that they could be effective in removing iron contaminants from such systems, in purifying (removing) solid ferric corrosion products, or in inhibiting calcium scale formation.

It is highly desirable to have a means of expelling and removing, dispersing and dissolving built-up iron corrosion products, while also preventing the formation of such products in systems in which an aqueous solution and iron or iron-based alloy materials are in contact with each other.

Summary of the invention

The present invention relates to a method for removing iron corrosion products from systems having a water/iron or iron-based alloy interface, particularly cooling water systems or heat exchanger systems, and therefore provides a means for ridding the systems of such products.

The present process requires the introduction and maintenance of at least one aromatic compound in the aqueous component of the system, said compound having two hydroxyl groups ortho-positioned, as adjacent-paired, with respect to each other and at least one electron-withdrawing group which is substituted for a hydrogen atom of the aromatic group.

Detailed description

The present invention relates to a method in which a soluble additive is used to clean cooling water systems of formed iron crusts.

It has been unexpectedly discovered that certain dihydroxyaromatic compounds which also contain electron-withdrawing substituents provide all of the desired properties set forth in the description of the background of the invention. The combination of paired hydroxyl groups and at least one electron-withdrawing group is essential to maintaining these dihydroxyaromatic additives as stable and soluble materials which have the ability to effectively remove the undesirable iron contaminant solids under cooling water conditions.

The present invention therefore provides a process for removing iron contaminants from an aqueous system comprising introducing into the system an aqueous solution containing a dihydroxyaromatic compound of the formula:

Q-(AR)-(OH)2

wherein Ar is a benzene aromatic radical, Q is at least one electron withdrawing group substituted on the Ar radical, and the hydroxyl (OH) groups on the Ar radical are substituted in ortho positions with respect to each other to provide a concentration of 0.1 to 50 ppm of the dihydroxy aromatic compound in the system, and removing portions of the aqueous system containing said iron impurities.

The invention further provides a method for removing solid ferric corrosion products from systems comprising a water/iron or iron-based alloy interface, which comprises introducing into the aqueous component of the system a solution comprising a dihydroxyaromatic compound as defined above to drive off and/or dissolve the corrosion products and removing portions of the aqueous system containing these corrosion products.

The compound to be used in the present process is described herein and in the appended claims as an aromatic compound having adjacent-paired dihydroxy groups or ortho-dihydroxy groups and at least one electron-withdrawing group directly bonded to the same aromatic moiety. The term "paired" or "ortho" as used herein and in the appended claims refers to the positioning of 2 hydroxy groups on adjacent carbon atoms of a single benzylic ring.

The compounds to be used in the process of the invention are aromatic compounds containing paired hydroxyl groups and at least one electron withdrawing group. The term "aromatic" as used in this specification and the appended claims is intended to refer to benzylic compounds unless otherwise indicated. The term "electron withdrawing group" as used herein and in the appended claims refers to a group having an electron withdrawing inductive effect known to enhance the positive charge and destabilize a carbonium ion of the aromatic group. Suitable electron withdrawing groups include -SO₃H, -SOR, -SO₂R, -NO₂, -F, -Cl, -Br, -CHO, -COR (e.g. -COCH₃), -CONH₂, -CONHR, -CONR₂, -CO₂H and -PO₃H₂. (where R = an alkyl group having 1 to 10 carbon atoms). The preferred groups are sulfonyl, carboxyl and nitro groups. Examples of the compounds considered are 3,4-dihydroxybenzenesulfonic acid (catechol-4-sulfonic acid), 4-nitro-1,2-benzenediol, 3,4- Dihydroxybenzoic acid, 4,5-dihydroxy-1,3-benzenedisulfonic acid (catechol-3,5-disulfonic acid) and salts of these acids. The salts are preferably formed with alkali and alkaline earth metals. The required compound corresponds to the formula:

Q - (Ar) - (OH)₂,

in which Ar is a benzene aromatic radical, Q is at least one electron withdrawing group which is substituted on the aromatic radical and the hydroxyl groups are bonded in a paired position to the benzene aromatic Ar group.

These aromatic compounds can be used in combination with known water treatment additives such as chelating agents, scale inhibitors, pH regulating agents, dispersants, biocides and/or corrosion inhibitors and mixtures thereof. Examples of chelating agents are N,N,N'N'-ethylenediaminetetraacetic acid and N,N'-bis(2-hydroxybenzyl)ethylenedinitrilo-N,N'-diacetic acid. Examples of pH regulating agents are mineral acids (e.g. H₂SO₄), organic acids (acetic acid), bases (e.g. NaOH) and various buffers (e.g. phosphates or borates). Examples of scale inhibitors are organophosphonates such as aminotrimethylenephosphonic acid and hydroxyethylidene-1,1-diphosphonic acid and polyacrylates. Examples of dispersants include carboxylate and sulfonate containing polymers. Examples of biocides are chlorine and bromine containing materials and quaternary ammonium salts. Examples of corrosion inhibitors suitable for use herein are inorganic acids (i.e., phosphoric acid), organic acids (i.e., citric acid, HEDPA) and salts of these acids such as phosphates, organophosphonates, chromates, molybdates and zinc salts.

The process according to the invention for removing iron-based scale in cooling water and similar systems comprises maintaining from 0.1 to 50,000 parts per 1,000,000 parts ("ppm"), preferably from 1 to 2,000 ppm, and most preferably from 5 to 200 ppm of at least one of the contemplated paired dihydroxy aromatic compounds (singly or multiple components) in the aqueous liquid; and removing portions of the aqueous system containing the iron contaminants. When the contemplated agents are used to accelerate clean (to rapidly clean a system containing corrosion), the agents are normally used in amounts of from 500 to 5,000 ppm and the system is maintained at a pH of from 6 to 9.5 (preferably 6 to 8). For example, after removal of iron contaminants, the system can be maintained free of iron contaminants by maintaining the compounds of interest in the system at concentrations of 2 to 20 ppm, with a pH of 7.5 to 9.5 (preferably less than 8). The temperature of the system being treated should be maintained at a temperature of from ambient to 93ºC (200ºF), and preferably up to 71ºC (160ºF). The composition used in the present invention can be added to the water by conventional by-pass feed means using briquettes containing the treating agent, by adding the compounds to the water either separately or together as dry powder mixtures, or it can be fed as an aqueous feed solution containing the treating components.

Another embodiment of the invention consists in the use of the contemplated paired dihydroxy aromatic compounds in combination with conventional calcium scale inhibitors as indicated above. Such scale inhibitors do not normally provide the desired inhibiting properties when the treated system contains iron contaminants. Conventional agents used to prevent and remove iron are again normally in Ineffective in the presence of calcium and known calcium crust inhibitors.

It has been unexpectedly found that conventional calcium scale inhibitors such as organophosphonates and polyacrylates function effectively and efficiently in the presence of the present paired dihydroxy aromatic agents when the system being treated contains iron contaminants. The aromatic agents under consideration can in turn demonstrate effective removal of iron contaminants when used in conjunction with calcium scale inhibitors. The calcium inhibitors and paired dihydroxy aromatic agents can be used in weight ratios of 1:50 to 50:1, preferably 1:10 to 10:1. The calcium inhibitor is most preferably maintained at 0.1 to 10 ppm in the liquid of the system being treated. The exact amount will depend on the amounts of each contaminant present and can be determined by routine experimentation.

The use of dihydroxy aromatic compounds containing electron withdrawing substituents (either alone or in combination with known cooling water additives) in aqueous systems controls iron scale in cooling water systems. The result is system clearance, maximized flow rates and heat transfer, and minimized corrosion and biological fouling.

The aqueous solutions used in the processes according to the invention comprise the dihydroxy aromatic compound (and optionally calcium scale inhibitor) and can consist essentially thereof.

The following examples are provided for illustration purposes only. All parts and percentages are by weight unless otherwise indicated.

Examples 1 to 10

A test water was prepared containing 99 parts per million (ppm) CaSO4, 13 ppm CaCl2, 55 ppm MgSO4, and 176 ppm NaHCO3. To 48.5 ml of this test water was added a solution of 5.1 mg FeCl3 6H2O in 1.5 ml water. While stirring vigorously, the pH was adjusted to 8.1 with NaOH (aq) and then stirred for 2 hours. This resulted in the precipitation of an iron-containing solid, presumably FeO(OH) nH2O. A second solution was prepared containing 100 ppm of the additive (as indicated in Table I) in 50 ml of the test solution at a pH of 8.1. The two solutions were combined and the mixture was stirred at 54°C for 17 hours. Then the mixture was filtered through a 0.1 µm membrane and the amount of dissolved iron was determined by atomic absorption. The results are shown in Table I. Examples 1 to 6 do not belong to the present invention but are given for comparison purposes. Table 1 Example Additive (50 ppm) Dissolved iron (ppm) without Ethylenediaminetetraacetic acid Diethylenetriaminepentaacetic acid Hydroxyethylidene-1,1-diphosphonic acid Aminotrimethylenephosphonic acid Catechol Catechol-4-sulfonic acid 4-Nitrocatechol Catechol-3,5-disulfonic acid 3,4-Dihydroxybenzoic acid

From the results given above, it is clearly evident that the process utilizing the compounds required by the present invention provides a far superior means of dissolving iron solids than representative known materials conventionally used for this purpose.

Examples 11 to 15

A solution of 50 ppm of the additive in the test water at pH 8.1 was combined with 1000 ppm Fe₂O₃ (hematite, particle size = 0 to 10 µm). This mixture was stirred for 17 hours at 54ºC and then allowed to settle undisturbed for 1 hour. A sample was removed at a depth of 50% and analyzed for iron by atomic absorption (after dissolution with HCl). The results are given in Table II. The Examples 11 to 13 do not belong to the present invention, but are given for comparison purposes. Table II Example Additive (50 ppm) Dispersed Fe₂O₃ (ppm) Without Diethylenetriaminepentaacetic acid Aminotrimethylenephosphonic acid Catechol-4-sulfonic acid Catechol-3,5-disulfonic acid

Example 16

Weighed mild steel coupons were pre-corroded by hanging vertically over an aerated saline solution (1% NaCl) for 24 hours. This procedure resulted in a deposit weight of 700 mg ± 15%. Two pre-corroded coupons were suspended in a tall beaker containing 900 ml of the test solution. The test solution, with a pH of 7.0 to 7.5, contained an additive according to Table III below, 500 ppm Ca2+, 100 ppm Mg2+ and was stirred at 500 rpm and 25ºC. At the end of this period, the coupons were removed and a 50 cc sample of the solution was taken. This sample was filtered through a 0.2 µm filter paper, acidified with 2 drops of 1:1 HCl and analyzed for total iron by atomic absorption. The steel coupons were dried at 100ºC for 1 hour and weighed. The coupons were then cleaned with inhibited HCl, washed with water and acetone and weighed again to determine the final weight of the deposit. The results of this test are reported as both soluble iron and percentage of deposit removal. Table III Additive Dose, ppm Deposit Removal % Soluble Fe, ppm Blank Catechol-4-sulfonic acid Catechol-3,5-disulfonic acid

Table III illustrates the ability of the compounds of the invention to remove rust in hard water.

Examples 17 to 19

All iron interaction tests for calcium carbonate threshold inhibitors were conducted in the following water: MgSO4 7H2O 266.1 ppm, CaCl2 234.9 ppm, CaSO4 ½H2O 199.9 ppm, Na2SO4 91.5 ppm, and NaHCO3 498.3 ppm.

The test solution was prepared by adding 1 ppm hydroxyethylidene-1,1-diphosphonic acid (HEDPA) to the above water in a 1000 ml beaker, followed by the specified amounts of additive according to Table IV and then again adding 1 ppm ferrous ion obtained from an aqueous solution of ferrous sulfate. The total volume of the solution was made up to 750 ml. The test solution was stirred at 500 rpm and heated to 60ºC in a water bath. The pH of the solution was monitored and maintained below 7.3 by adding dilute HCl. After the required temperature was reached, 0.066 N NaOH was added at a rate of 0.3 ml/min using an automatic titrator.

The pH was monitored and recorded during the titration. When calcium carbonate began to precipitate, a decrease or a plateau in pH was observed. This point was referred to as the critical pH. Threshold inhibitors such as HEDPA function to increase the critical pH and therefore decrease the tendency of the water to form scale. When iron interacts (interferes) with the HEDPA, a decrease in threshold activity is observed due to a decrease in critical pH. The results are summarized in Table IV. Example 17 is not part of the present invention but is provided for comparison purposes. Table IV Critical pH for an additive concentration of: Example Additive Ethylenediaminetetraacetic acid Catechol-4-sulfonic acid Catechol-3,5-disulfonic acid

HEDPA gave a critical pH of 9.10 in the absence of ferrous ions and a critical pH of 8.70 in the presence of iron ions, indicating a significant loss of threshold activity. Therefore, a critical pH of 8.7 indicates 0% activity by the specified additive for removing the iron interference, while a critical pH of 9.10 indicates 100% activity.

As shown in Table IV, ethylenediaminetetraacetic acid, a commonly used chelating agent, is completely ineffective in eliminating the iron interaction of HEDPA, which is a calcium carbonate crust inhibitor. On the other hand, the addition of catechol-4-sulfonic acid or catechol-3,5-disulfonic acid at 5 ppm restored most of the threshold activity of HEDPA.

Claims (12)

1. A process for removing iron contaminants from an aqueous system, which comprises introducing into the system an aqueous solution containing a dihydroxyaromatic compound of the formula: Q - (Ar) - (OH)₂ wherein Ar is a benzene aromatic radical, Q is at least one electron withdrawing group substituted on the Ar radical, and the hydroxyl (OH) groups on the Ar radical are substituted in ortho positions with respect to each other to provide a concentration of from 0.1 to 50,000 ppm of the dihydroxyl aromatic compound in the system, and removing portions of the aqueous system containing said iron impurities. 2. The process of claim 1 wherein the system is maintained at a pH of less than 9.5 and a temperature of less than 93ºC (200ºF). 3. A process according to claim 2, wherein the dihydroxy aromatic compound is introduced into the system to provide a concentration therein of 500 to 5000 ppm and wherein the system is maintained at a pH of 6 to 9.5. 4. A process for removing solid iron(III) corrosion products from systems comprising a water/iron or iron-based alloy interface, which comprises introducing into the aqueous component of the system an aqueous solution containing a dihydroxyaromatic compound of the formula: Q - (Ar) - (OH)₂ wherein Ar is a benzene aromatic radical, Q is at least one electron withdrawing group substituted on the Ar radical, and the hydroxyl groups are in ortho positions with respect to each other to provide a concentration of 0.1 to 50,000 ppm of the compound in the system, contacting the aqueous component with the solid ferric product to drive off and/or dissolve that product, and removing portions of the aqueous system containing those corrosion products. 5. The process of claim 4 wherein Q is -SO₃H, -SO₂R, -COOH, -NO₂, -F, -Cl, -Br, -CHO, -COR, -CONH₂, -CONHR, -CONR₂, -PO₃H₂, a metal sulfonate salt or a metal carboxylate salt and R is an alkyl group. 6. A process according to any preceding claim, wherein the aqueous system is rich in calcium and further comprising introducing a calcium scale inhibitor into the aqueous system in a weight ratio of 1/50 to 50/1 of scale inhibitor to dihydroxy aromatic compound. 7. The method of claim 6, wherein the scale inhibitor is an organophosphonate or a polyacrylate. 8. A process according to claim 6 or 7, wherein the scale inhibitor and the dihydroxy aromatic compound are present in a weight ratio of 1:10 to 10:1. 9. A method according to any preceding claim, wherein the dihydroxy aromatic compound is introduced into the system to provide a concentration of 1 to 2000 ppm of the dihydroxy aromatic compound in the system. 10. A process according to any preceding claim, wherein Q is -SO₃H, -COOH, -NO₂, a metal sulfonate salt or a metal carboxylate salt. 11. The process of claim 10, wherein the dihydroxyaromatic compound is 3,4-dihydroxybenzenesulfonic acid, 3,4-dihydroxybenzoic acid, 4,5-dihydroxy-1,3-benzenedisulfonic acid, a salt of one of these acids, or 4-nitro-1,2-benzenediol. 12. A method according to any preceding claim, wherein the aqueous component of the system further comprises a water treatment agent which is a chelating agent, a scale inhibitor, a pH regulator, a dispersant, a corrosion inhibitor or a biocide.